Multiple access (MA) schemes are used to simultaneously share available bandwidth among a plurality of users in a communication system. In conventional MA applications, a finite and contiguous amount of spectrum is designated for a particular system. The use of the spectrum is typically considered unconstrained.
In recent years, however, the increasing demand for ubiquitous computing has resulted in a radio spectrum crowded with users seeking reliable and high bit-rate communication. For example, the Unlicensed National Information Infrastructure (UNII) band at 5.2 GHz contains a wide variety of signals such as microwave ovens, RF tags, cordless telephones, Wireless Local Area Networks (WLANs), Wireless Personal Area Networks (WPANs), etc.
The MA problem has evolved, therefore, to include not only the sharing of spectrum among users of a single system but also the sharing of spectrum among multiple, frequency-adjacent systems. In other words, a MA wireless system must frequently coexist in frequency with numerous, disparate, and uncoordinated interferers.
This coexistence requirement necessitates that a system's signal be spectrally-shaped to limit its interference to signals of wireless systems that exist in the same frequency band.
One common spectrum shaping approach is based on orthogonal frequency division multiple access (OFDMA). Orthogonal frequency division multiplexing (OFDM) distributes data over a large number of OFDM carriers that are spaced apart at precise frequencies.
Accordingly, spectrum shaping in OFDMA is achieved by turning off a set of OFDM carriers at frequencies occupied by existing signals or at which strong interference is present. In practice, however, existing signals or interference are not necessarily centered at OFDM carriers, but can spectrally leak into adjacent frequency bands forcing a large number of the carriers to be turned off. This spectral leakage problem can severely impact the capacity of an OFDMA-based spectrum shaping approach.
What is needed therefore is an efficient spectrum shaping technique for multiple access communication in constrained spectrums.